1. Field of the Invention
The present invention relates to microwave ovens, and more particularly, to a microwave oven including an antenna for radiating microwaves which realizes uniform heating in a cavity.
2. Description of the Background Art
A conventional microwave oven is disclosed in Japanese Patent Laying-Open No. 62-295386, for example. In this conventional microwave oven, microwaves generated from a magnetron are propagated via a waveguide into a cavity in which a food which is the substance-to-be-heated is accommodated. FIG. 1 is an exploded perspective view showing a structure of a waveguide of such a conventional microwave oven.
Referring to FIG. 1, microwaves generated from an output antenna 101 of a magnetron not shown are propagated within a rectangular waveguide 102. At a sidewall of waveguide 102 in contact with a cavity not shown, a projection portion 103 for coupling having substantially a truncated cone configuration is provided projecting inward the waveguide. A coupling aperture 104 is provided at the center of the top portion of projecting portion 103. A cylindrical radiation antenna 105 is held by a cover 106 of a dielectric material so as to pass through coupling aperture 104.
Such a conventional microwave oven has microwaves emitted into a cavity not shown due to an electric field generated between cylindrical radiation antenna 105 and the sidewall of coupling projection portion 103. The direction of emission is unitary at right angles to the electric field. Because the cross sectional area of the cavity is extremely greater than the opening area of coupling projecting portion 103 at the cavity side, unidirectional microwaves provided to the cavity do not spread out widely within the cavity, resulting in unevenness in heating.
To realize uniform heating within a cavity, a microwave oven including a flat radiation antenna directly secured to the antenna of a magnetron with a screw is disclosed in Japanese Utility Model Publication No. 53-50122, for example. FIG. 2 is an exploded perspective view showing structures of a magnetron and a radiation antenna of such a conventional microwave oven.
Referring to FIG. 2, a flat radiation antenna 113 is attached to an antenna 112 of a magnetron 111 by a screw 114 or the like. A plurality of slits 115 are formed in radiation antenna 113 for enabling energy emission in various modes.
FIG. 3 schematically shows a manner in which microwaves are propagated according to the structure shown in FIG. 2. Referring to FIG. 3, microwaves (indicated by open arrow) are emitted at right angles to an electric field (indicated by general arrow) generated between a cavity 116 and a radiation antenna 113. This range of radiation is limited to that indicated by the broken lines in FIG. 3. This means that nonuniform heating occurs within the cavity.
Furthermore, because radiation antenna 113 is secured to antenna 112 of magnetron 111 by screw 114, a gap may be formed between antenna 112 and radiation antenna 113 as a result of vibration during usage or by insufficient tightening of screw 114 when radiation antenna 113 is reattached after repairment or the like to result in a possibility of a spark occurring in the gap.